The present invention relates to electrical heating elements and is concerned in particular with electrical resistance heating elements, principally for use in domestic appliances which involve the heating of liquids for food preparation such as kettles, heating jugs, coffee percolators and the like, and are of the type which do not intrude into the volume of liquid to be heated.
Conventional electrical liquid heating elements fall into two general categories.
The first category comprises sheathed elements consisting of a metal tube along the longitudinal axis of which is situated a conventional spiralled wire element and which in use an oxide as a means of providing dielectric (electrical insulation) between the tube and spiralled element. These sheathed elements are generally formed into some form of loop or spiral and are situated in the bottom of a vessel designated for liquid heating. As such they intrude into the volume of the liquid to be heated.
The latter elements are generally used for heating only one liquid, almost always water, as their convoluted shapes make them difficult to clean and completely remove traces of one type of liquid, should they be required to be used with a second.
In addition, the need to have an outer metal sheath to separate the spiralled wire element from the liquid and to have a dielectric oxide filling to separate metal sheath from element, have the result that such elements are of relatively large mass and low surface area, which combine to reduce their operational efficiency in heating liquids.
The second category of known elements comprises those which consist of a flat plate, forming the base of the heating vessel, through which heat flows from element to liquid. Such elements do not intrude into the volume of liquid to be heated.
This second category of element may be subdivided into two types, namely, those which simply use a conventional sheathed element fixed to the back of a flat plate, which then acts as a heat sink, and a second type which may be classified generally as thick film resistive heating elements.
The conventional approach to the formation of thick film elements is to utilise a metal substrate, onto the surfaces of which is applied a dielectric coating, usually a glaze. Screen printing techniques are employed to deposit an ink, consisting of a solvent and a mixture of metals and/or metal oxides, to one coated surface in the form of an element configuration comprising one or more printed circuit conductive tracks. The printed item is then fired to drive off the solvent and to melt the resistive particles of metal and/or oxide. A final dielectric coating, usually a glaze, is then applied to the screen printed element configuration to act as a protective layer.
Whilst these conventional sheathed and screen printed thick film elements may be very adequately used to heat liquids, they are subject to various constructional and operational disadvantages, some of which are listed as follows.
Because of the need to use an oxide dielectric filler in sheathed elements, the spiralled resistive wire which generates the heating effect is required to run at temperatures well in excess of those required to boil liquids. As a result, such elements are very prone to overheating and burn-out if operated without sufficient volume of surrounding liquid. In addition, their relatively high thermal mass detracts from their operational efficiency, as a large proportion of the heat initially generated goes directly into raising the temperature of the dielectric metal oxide and metal sheath and not into the liquid. This reduces the liquid heat-up rate.
In order to utilise sheathed elements in a flat plate configuration, it is necessary to combine then into another supporting metal plate, or layer. This plate, or layer, is usually of aluminium and serves as a heat sink, in effect providing a larger surface area over which the sheathed element may dissipate the heat energy being generated. The combination of aluminium plate, or layer, is then attached to the metal plate forming the base of the heating vessel. Whilst increasing the heat dissipating area of the sheathed element, this aluminium plate substantially increases the thermal mass of the system, which in turn detracts from the operational efficiency as it requires more energy initially to preheat it, before heat is transferred to the liquid.
The combination of sheathed element and aluminium layer, or plate, is also prone to operational failure where there is inadequate attachment of the sheathed tube to the aluminium plate. At any points of inadequate attachment, the heat being generated by the sheathed element cannot be fully dissipated to the aluminium plate acting as a heat sink. As a result, the temperature of the sheathed element at such points may rise to quite high levels. The localised thermal expansion associated with these "hot spots" may result in element failure or a progressive detachment of the element from the aluminium plate, which serves to exacerbate the over-heating problem and accelerate element failure.
It is also known that there are constructional and operational problems associated with the existing screen printed thick film electrical resistance heating elements, which may be summarised as follows.
(a) Variations in the thickness or consistency of the conductive/resistive ink, as applied during the screen printing process, will result in unevenness in the final resistive element track. Such localised unevenness may result in the generation of "hot spots" within the elements track, leading to failure in operation.
(b) The presence of any defects or holes in the final protective glaze layer, such as those due to the presence of solvent traces, allow the resistive tracks to be locally oxidised, forming localised hot spots and leading to track failure.
(c) The screen printed elements are of a tracked form, usually spiralled. The tracks are discrete and usually are subdivided into parallel paths, and so configured as to cover the maximum amount of substrate area as possible. Despite this configuration, only a relatively small proportion of the substrate area is actually covered by the element in practice and in consequence the operating temperatures need to be well above the boiling points of the liquids being heated in order to achieve good heat transfer through the substrate.
(d) Another factor which deleteriously affects heat transfer from element to liquid is the combination of the metal substrate and the glazed insulating layer. Generally, the metal substrate which has been used is stainless steel, which has a poor coefficient of heat transfer when compared with say copper or aluminium.
The present invention seeks to overcome or substantially reduce the problems described above associated with the known configurations and manufacturing techniques.